The present invention relates to a dust core used as magnetic cores for transformers, inductors, etc., cores for motors, and used for other electromagnetic parts, a powder used for the fabrication of the dust core, and a process for the fabrication of the dust core.
Recent size reductions of electric, and electronic equipment have resulted in the need of small-size yet high-efficient dust cores. For a dust core, ferrite powders, and ferromagnetic metal powders are used. The ferromagnetic metal powders are higher in saturation magnetic flux density than the ferrite powders, and so enable core size to become small. However, low electric resistance gives rise to an increase in the eddy current loss of the resulting core. For this reason, insulating coatings are usually provided on the surfaces of ferromagnetic metal particles in the dust core.
In an ordinary dust core fabrication process, annealing is generally carried out after molding because coercive force is increased by stresses induced during molding, resulting in a failure in obtaining high permeability and an increased hysteresis loss. To provide sufficient release of stresses from ferromagnetic metal particles, they must be annealed at a high temperature (of, e.g., 550.degree. C. or higher). So far, phenol or silicone resin having relatively high heat resistance has often been used as an insulating material. Even when these resins are used, however, insulation between the ferromagnetic metal particles becomes poor because of increased resin losses upon a thermal treatment at 550.degree. C. or greater. The poor insulation in turn gives rise to some noticeable eddy current losses in a high-frequency region, resulting in increased core losses and causing the dependence of permeability on frequency to become worse.
An object of the present invention is to achieve a dust core having high saturation magnetic flux density, low losses, and satisfactory permeability with its dependence on frequency being improved.